A wind turbine known in the art comprises a wind turbine tower and a wind turbine nacelle positioned on top of the tower. A wind turbine rotor with a number of wind turbine blades is connected to the nacelle through a low speed shaft, as illustrated in FIG. 1.
One of the consequences of producing power by means of a wind turbine is the generation of noise from the mechanical system (gear, generator, cooling fans, etc.) and the aerodynamic noise from the blades. With off-shore wind turbines, the noise emission is not necessarily an issue but with land based wind turbines noise emission can be a problem if not carefully controlled.
Two of the most important parameters that influence the magnitude of noise generated by the wind turbine are the angular speed of the blade and the angle of attack of the incoming wind that passes the rotating blade.
One way of ensuring an acceptable noise level at locations neighboring the wind turbine is to measure the noise level under given circumstances and then control the operation of the wind turbine (e.g. the rotation speed of the rotor or the blades angle of attack) to ensure that the critical noise level is never reached. But the noise level at a given position near the wind turbine depends on a large number of factors, which often interact, and it is therefore necessary to operate with relatively large margins of safety to the critical noise level to ensure that the critical level is not exceeded under any circumstances.
From U.S. Pat. No. 6,688,841 it is also known to more or less constantly measure the sound level at a location close to a wind turbine park and then reduce the rotational speed of the rotor if a predefined level is exceeded. However, such sound level measurements are subject to uncertainty in that a large number of factors can affect the measurements such as background noise, rain, wind or other and it can be very complex and expensive to establish and operate this system if the noise level at many different locations has to be taken into consideration.
An object of the invention is therefore to provide an advantageous and cost-efficient technique for operating a wind turbine.